Surface enhanced electron correlation on the trivial quasi-two-dimensional bulk insulator 1T−TaS2

Abstract

While the prototypical quasi-two-dimensional charge density wave system of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ has been known as a Mott insulator with a possibility of quantum spin liquid, recent band-structure calculations and spectroscopic works in parallel suggested a metallic system or a spin-singlet insulator due to the interlayer coupling. Here, we carefully reinvestigate the out-of-plane electron dispersion, which reflects the interlayer electronic coupling, with angle-resolved photoelectron spectroscopy. We identify two distinct branches for the topmost valence band, which can be unambiguously related to the surface and the bulk layers with different band gaps. Density functional theory calculations clearly indicate a trivial band insulator due to the interlayer coupling for the bulk but the surface band gap affected substantially by the electron correlation. The surface-bulk electronic dichotomy consistently incorporates most of the theoretical and spectroscopic results reported so far and has wide implications for van der Waals materials with nontrivial interlayer interactions.